Conventionally, a reforming apparatus, that performs steam reformation of a raw material to be reformed and produced a reformed gas containing hydrogen as a principal component, has been known. One of applications of the reformed gas is a fuel utilizable to generate electricity in a fuel cell but, in this case, since the carbon monoxide is poisonous to electrodes of the fuel cell, it is desired that the content of carbon monoxide (CO) in the reformed gas should be removed to a level of 100 ppm or less. Therefore, CO is removed from the reformed gas by employing, after the step of steam-reforming the raw material, a step of decreasing the concentration of CO in the resultant reformed gas by water-gas-shift reaction and a step of further decreasing the concentration of CO in the resultant reformed gas by selectively oxidizing CO, as disclosed in JPA HEI 5-251,104. However, conventionally, since the three reaction steps mentioned above are performed separately in respective apparatuses, a reforming system as a whole tends to be bulky. In addition, since heat sources for providing heat of reaction are needed separately in respective reaction steps, heat loss is large. Therefore, in the conventional reforming apparatus, it has been desired to lower the heat loss and to reduce the size.
On the other hand, a prior art reforming apparatus designed to reduce the size thereof is disclosed in JPA HEI 7-126,001. Referring to this reforming apparatus, the reforming apparatus includes a reformation treating layer having a reforming reaction unit or section, a shift reaction unit and a CO oxidation unit, all arranged in series with each other along the direction of flow of gas, and a combustion gas flow path layer through which combustion gas coming from a combustion part passes, and has a structure in which the reformation treating layer and the flow path layer are alternatively positioned side by side above the combustion part. Thus, since the above three reaction units in this reforming apparatus are provided with heat from the combustion gas flow path layer, the reforming apparatus can make good use of heat in the combustion part. Also, since the reforming apparatus has the three reaction units built in one apparatus, downscaling is easy to accomplish.
However, the prior art reforming apparatus involves a problem that the temperature in each of the reaction units can not be suitably controlled. That is, it is known that catalytic reactions takes place in all of the above three reaction units and that there is a range of reactive temperature required in each of the reactions that take place respectively in the three reaction units. The range of reactive temperature of steam reforming reaction, which varies according to the kind of the raw material, for example, is about 400 to 1000° C., preferably, 600 to 900° C., when the raw material is a hydrocarbon such as butane, and also 250 to 400° C. when the raw material is methanol. On the other hand, the range of reactive temperature required by the water-gas-shift reaction or the CO selective oxidation reaction does not vary so much according to the kind of the raw material, and the range of reaction temperature required by the water-gas-shift reaction is generally about 200 to 350° C. and, preferably, 220 to 300° C., and that required by the CO selective oxidation reaction is generally about 100 to 250° C., preferably, 120 to 180° C. In general, the range of reactive temperature decreases in the order of that in the reforming reaction unit, that in the shift reaction unit, that in the CO oxidation unit. Therefore, it is necessary to control the temperature in each reaction unit so as to be in the above respective range of reactive temperature. However, the reforming reaction unit and the shift reaction unit in the above prior art reforming apparatus do not separate from each other, but continued unitarily, that is, they are functionally distinguished in that the form of reaction changes from the steam reforming reaction onto the water-gas-shift reaction as the temperature of the reformed gas lowers. For this reason, even though this reforming apparatus is capable of effectively performing the steam reformation of methanol which requires a small difference between the reforming temperature and the shift reaction temperature, the steam reformation of hydrocarbons tends to exhibit a temperature diverting from the required temperature range during a transit from the reforming unit to the shift reaction unit and, therefore, a problem would arise with the hydrocarbon such as butane of which reactive temperature range during the steam reforming reaction is high. Also, since the prior art reforming apparatus has a laminated structure that the reformation treating layers and the combustion gas flow path layers alternate sidewise, the same reaction units tend to have a varying temperature depending on the position in the laminated structure, and specifically, a temperature difference between a position near to the outer periphery of the apparatus and a center position of the apparatus tends to be considerably large because the position near to the outer periphery of the apparatus is cooled by the air outside. Particularly, this varying temperature becomes problematic in the CO oxidation unit which has a narrow range of reactive temperature. Thus, when it occurs that some of the reaction units have a temperature diverting from the required temperature range, there is a fear that the hydrogen content in the resultant reformed gas lowers and the CO concentration would not be sufficiently lowered.